The present invention relates to calibration of a radiation detection medium in a radiotherapy treatment system, and in particular, to methods for calibrating a radiation detection medium in a radiotherapy treatment system wherein the calibration is determined by exposing arbitrary areas of the medium to a plurality of dose levels.
Radiotherapy has been used for years as a method for irradiating and selectively killing cancer cells while minimizing radiation exposure to adjacent tissue. The effectiveness of radiotherapy can depend upon the absorbed dose or the amount of energy deposited within a tissue mass. Absorbed dose is typically measured in centigray or cGy units.
A radiation detection medium may be used to determine the amount of radiation to which a patient is subjected during radiation treatment. Particularly useful is a two-dimensional or three-dimensional radiation detection medium that can determine radiation dose over an area or within a volume. Examples of the former are radiographic film, radiochromic film, phosphor plates, two-dimensional arrays of diodes or ion chambers, electronic portal imaging devices (EPID) and the like. An example of a volume detector is a 3-dimensional radiation sensitive gel such as “BANG” gel and the like. The radiation detection medium typically has a response that varies systematically in accordance with the degree of radiation exposure. Radiation detection media such as radiographic and radiochromic films and 3-dimensional gels typically have a light transmission or optical density that varies systematically in proportion to the radiation dose. Calibration of the radiation sensitive film or gel detection medium allows one to measure the absorbed dose indirectly by measuring the light transmission or optical density of the exposed radiation detection medium.
Calibration curves for a radiation detection medium are often prepared by exposing one or more areas of the detection medium to different and known amounts of radiation using a linear accelerator or a similar device capable of generating a range of known dose levels. Another method frequently used is to expose the detection means to a continuously varying level of doses. This can be done by interposing a wedge of material with continuously varying thickness between the radiation source and the detection medium. Alternatively, the radiation sensitive medium may be sandwiched between two blocks and positioned so that the medium is in a plane parallel to the beam when it is exposed. In this configuration the dose applied to the radiation sensitive medium decreases continuously with depth below the top surface of the blocks. This type of exposure is often referred to as a depth-dose exposure. Typically, calibration curves are generated by measuring the response of the radiation sensitive medium for numerous different dose levels. In the instance of radiation films or gels, it is common to measure the light transmission or optical density of the medium for numerous different radiation dosage levels.
In accordance with one method for calibrating a radiation detection medium described in U.S. Pat. Nos. 6,934,653 and 6,675,116 to Ritt, a radiation detection medium is divided into a number of predefined regions, each predefined region is exposed to a different selected dose and the plurality of responses in the detection medium are measured to generate a calibration. In accordance with one aspect of the described process, measurement of the plurality of responses is effected through the use of computer software to automatically obtain measurements in the predefined areas. However, there are certain drawbacks to a calibration based on exposing predefined regions of the radiation detection medium. Since the radiation detection medium is exposed in predefined regions, it is essential to accurately align and position the radiation detection medium with respect to the radiotherapy treatment system. As the radiation detection medium is placed in position by a combination of manual and visual means, it can be difficult to maintain precise registration of the film to the radiotherapy treatment system. Another source of possible error in the use of predefined areas for generating a calibration curve is related to reading and measuring of the radiation detection medium using a film scanner. In order for this to be effective and produce reliable results, the radiation detection medium must be registered in a known and reproducible location with respect to the film scanner. Accordingly, any mis-registration of the radiation detection medium with respect to either the radiotherapy treatment system or the film scanning system will typically lead to an error in measurement of the exposed areas in a system that automatically takes measurements in the predefined areas.
Yet another source of possible error in the use of pre-defined areas of exposure can result from mis-orientation of a radiation detection medium on a scanner. In particular, if the pre-defined areas do not form a symmetric pattern on the detection medium then the pattern of predefined areas can only be reproduced if the film is placed on the scanner in the correct orientation. This is explained with reference to FIG. 1 which depicts a radiation sensitive medium with four areas shown at 1, 2, 3 and 4 exposed to different doses of radiation. Two axes of the radiation sensitive medium are shown at X and Y. A third axis, the Z axis, not shown in FIG. 1, runs perpendicular to the X and Y axes, directly into the plane of the figure. FIGS. 2, 3 and 4 depict how the pattern of exposures appears after the radiation sensitive medium has been rotated by 180° about the X, Y and Z axes respectively. Inspection of these patterns indicates that each is unique and demonstrates that the radiation sensitive medium must be correctly oriented with respect to rotation about these axes for the pattern of exposures to appear in pre-determined locations when the radiation sensitive medium is positioned for scanning.
Accordingly, there is a need for a method of calibrating a radiation detection medium to address the foregoing issues with the prior art. More specifically, there is a need for a method capable of generating a calibration for a radiation detection medium that is not limited to making measurements in predefined areas.